All of a sudden I am unable to send newsletters' out to customers. I can manually send email, both Plain Text & HTML. Invoices are going out and several test GV Mails also go out as expected.

Any idea what's going on?

I checked with the ISP and nothing has changed according to them.

I'm on a Linux Server
zencart is v.1.3.9h
Apache is 2.2.17
PPHP Is 5.2.15
MySQL is 5.1.52
zencart Admin Directory name has been changed.

HELP!